1. Field of the Invention
This invention relates generally to quadrature modulation systems for transmitting analog signals and more particularly to the application of quadrature modulation techniques to increasing the dynamic amplification range of such analog signals.
2. Description of the Prior Art
In quadrature modulation systems of the prior art two independent baseband, i.e., having frequency components extending to zero Hertz, signals are individually amplitude modulated with two carrier waves having the same frequency, but relative quadrature, i.e., separated by ninety electrical degrees, phases. The two resultant amplitude-modulated (AM) signals are subsequently combined to yield a single double-sideband AM signal which contains and preserves the information in the two independent baseband channels.
At the receiver the reconstruction of the individual baseband channels can be accomplished by precisely generating the two out-of-phase carriers and using them to demodulate the received double-sideband signal.
The objective of this quadrature modulation arrangement is to conserve frequency space in the transmission medium. A quadrature modulation system, as is well known, has twice the channel capacity in the same bandwidth as the single-channel double-sideband AM system.
In many telemetry systems, such as are used in geological and underwater survey systems, acoustical signals are employed to determine the nature of subterranean and submarine characteristics. There are in general two principal types of sounds encountered in either of these environments--ambient noise and calibrated impulsive or projector sounds for measuring transmission loss through rock strata or waters of varying salinity and temperature gradient. Ambient noise, with particular regard to submarine environments, arises from surface turbulence, biological noise, ships and oil rigs. These noises are passive from the viewpoint of the observer, i.e., the observer has neither control over, nor foreknowledge of, their occurrences. Transmission loss is measured in both subterranean and submarine environments from calibrated explosive charges or high-energy projector tones.
Ambient noise occurs at very low levels and produces responses in the tenths and hundredths of millivolts. Responses from explosive charges reach tens and hundreds of millivolts. The dynamic volume range to be observed thus extends upward to 90 decibels (DB). It is difficult and expensive to design and construct a single compact broadband amplifier and modulators with so expansive a dynamic range.
It is accordingly an object of this invention to provide a simplified amplifying system with a wide dynamic range.
It is another object of this invention to adapt a quadrature modulation arrangement to increase the dynamic range of baseband signals which can be transmitted by analog means.
It is a further object of this invention to provide a wide dynamic volume range over a coaxial cable by using quadrature-modulation principles.